Environmental Engineering Reference
In-Depth Information
volcanic dust, which for a number of reasons is
particularly suitable for such studies. For
example, the source of the aerosols can be
easily pin-pointed and the volume of material
injected into the atmosphere can often be
calculated; the dust includes particles from a
broad size-range and it is found in both the
troposphere and the stratosphere. Recent
studies, however, have suggested that the
sulphate particles produced during volcanic
eruptions have a greater impact on the energy
budget than volcanic dust and ash.
October of 1991, the cloud was present above
Resolute at 74°N in the Canadian Arctic, and
by early 1992 had spread worldwide (Rosen
et
al.
1992). Sulphate aerosols and fine ash
particles from Mount Hudson which erupted in
Chile in August 1991 were carried by strong
zonal westerlies between 45°S and 46°S to pass
over southeastern Australia within 5 days of
the eruption, and around the earth in just over a
week (Barton
et al.
1992).
The build up of the dust veil and its eventual
dispersal will depend upon the amount of
material ejected during the eruption and the
height to which the dust is projected into the
atmosphere. The eruption of Krakatoa released
at least 6 cubic km (and perhaps as much as 18
cubic km) of volcanic debris into the atmosphere
(Lamb 1970). In comparison, Mt St Helens
produced only about 2.7 cubic km (Burroughs
1981). Neither of these can match the volume of
debris from Tambora, an Indonesian volcano
which erupted in 1815, producing an estimated
80 cubic km of ejecta (Findley, 1981). More
important than the total particulate production,
however, is its distribution in the atmosphere.
That depends very much on the altitude to which
the debris is carried, and whether or not it
penetrates beyond the tropopause. The maximum
height reached by dust ejected from Krakatoa
has been estimated at 50 km and a similar altitude
was reached by the dust column from Mount
Agung in 1963 (Lamb 1970). A particularly
violent eruption at Bezymianny in Kamchatka,
in 1956, threw ash and other debris to a height
of 45 km (Cronin 1971), but Mt St Helens,
despite the explosive nature of its eruption, failed
to push dust higher than 20 km, perhaps because
the main force of the explosion was directed
horizontally rather than vertically (Findley 1981).
As a result, it has been estimated that Mt St
Helens injected only 5 million tonnes of debris
into the stratosphere compared with 10 million
tonnes for Mount Agung, and as much as 50
million tonnes for Krakatoa (Burroughs 1981).
The eruptions of El Chichón in 1982 and Mount
Pinatubo in 1991 injected an estimated 20 and
30 million tonnes of material respectively into
VOLCANIC ERUPTIONS AND
ATMOSPHERIC TURBIDITY
The large volumes of particulate matter thrown
into the atmosphere during periods of volcanic
activity are gradually carried away from their
sources to be redistributed by the wind and
pressure patterns of the atmospheric
circulation. Dust ejected during the explosive
eruption of Krakatoa, in 1883, encircled the
earth in about two weeks following the original
eruption (Austin 1983), and within 8 to 12
weeks had spread sufficiently to increase
atmospheric turbidity between 35°N and 35°S
(Lamb 1970). The diffusion of dust from the
Mount Agung eruption in 1963 followed a
similar pattern (Mossop 1964) and in both
cases the debris eventually spread polewards
until it formed a complete veil over the entire
earth. The cloud of sulphate particles ejected
from El Chichón in 1982 was carried around
the earth by the tropical easterlies in less than
20 days, and within a year had blanketed the
globe (Rampino and Self 1984). Similarly, the
volcanic debris injected into the atmosphere
during the eruption of Mount Pinatubo in June
1991, circled the earth at the equator in about
23 days (Gobbi
et al.
1992). It reached Japan
two weeks after the eruption began (Hayashida
and Sasano 1993). Within 20 days the edge of
the debris cloud had reached southern Europe
(Gobbi
et al.
1992), and less than 2 months
later was recognized in the stratosphere above
southern Australia (Barton
et al.
1992). By
